Cobalt is a rare metal and is a valuable metal used as a material of an alloy. Cobalt is also used as an electrode material for batteries other than alloys. For example, cobalt is also used for a positive electrode material of a lithium ion battery, which is a nonaqueous electrolyte secondary battery for in-vehicle use that has been developed recently.
When producing a positive electrode material of the lithium ion battery which is the nonaqueous electrolyte secondary battery, a metal hydroxide called a precursor prepared by neutralizing an aqueous solution of a metal salt mixed at a predetermined ratio is generally formed. When the precursor and a lithium compound are mixed and fired, a positive electrode material is produced. In production of a positive electrode material containing cobalt, a salt containing cobalt (for example, a cobalt salt such as cobalt sulfate or cobalt chloride) is used when the above-described aqueous solution of a metal salt is produced.
The above-described cobalt salt can be obtained as a by-product in a process of smelting nickel ore and the like. Specifically, wet processing is employed for refinement of impurities, and a cobalt salt is generated from a cobalt salt solution produced at the processing. However, in addition to nickel and cobalt, nickel ore and the like contain various kinds of impurities such as manganese, iron, copper and chromium. If the cobalt salt solution also contains impurities, the impurities may be mixed into the cobalt salt. If the cobalt salt containing impurities is used for producing the positive electrode material, impurities may be mixed into the positive electrode material.
The presence of impurities in the positive electrode material greatly affects the performance of the positive electrode material, that is, battery characteristics. In particular, since the lithium ion battery as described above has a high capacity and a high voltage, the presence of a trace amount of impurities greatly affects the battery characteristics, and therefore, specifications of impurities of raw materials such as a cobalt salt are controlled extremely strictly. In particular, since copper is an important impurity which greatly affects the performance of the battery, it is required to strictly control the amount of copper contained in raw materials such as a cobalt salt.
As a method for reducing impurities such as copper contained in the cobalt salt, methods such as a solvent extraction method and an electrolytic method have been known. In other words, when copper is removed from the cobalt salt solution by the solvent extraction method or the electrolytic method, the copper concentration in the cobalt salt solution, that is, the amount of copper contained in the cobalt salt can be reduced. However, these methods cannot significantly lower the lower limit concentration of separable copper. In addition, these methods require a large scale apparatus such as a solvent extraction apparatus such as a mixer settler, an electrolytic cell, and a power source, so that there is a problem that the cost of equipment investment increases and the processing cost increases.
As a simpler method than the solvent extraction method and the electrolytic method, there is a precipitation method. In the precipitation method, a neutralizing agent, a sulfurizing agent and the like are added to generate a precipitate and to separate impurities, and this method has been widely used for wastewater treatment for heavy metals such as copper and the like.
In a sulfurization method of precipitating and removing copper as a sulfide using a sulfurizing agent, the solubility of copper sulfide is very small (water solubility: 18° C., 3.4×10−4 g/L), and there is an advantage that the concentration of copper in a solution can be greatly reduced. However, since a harmful hydrogen sulfide gas is used as a sulfurizing agent, securement of the safety of workers and environmental measures are required. Although various efforts have been made to control hydrogen sulfide (for example, Patent Document 1), the device configuration is complicated, so that there is a problem that the cost for an incidental facilities increases.
It is also conceivable to employ a neutralization precipitation method in which an alkali such as sodium hydroxide is added to form hydroxide precipitates of heavy metals and remove them. In the case of removing copper by the neutralization precipitation method, the pH of a solution is usually adjusted within the range of pH 8 to 12 from the viewpoint of solubility (for example, Non-Patent Document 1). However, since cobalt also precipitates in the same pH range, cobalt precipitates together with copper when the neutralization precipitation method is used for a cobalt salt solution, and cobalt is lost. In order to reduce the loss of cobalt, it is conceivable to remove copper in a pH range lower than the above range. Although it is possible to precipitate copper hydroxide at a pH less than pH 8, the solubility increases. Thus, the concentration of copper in the cobalt salt solution cannot be significantly reduced. Specifically, in order to prevent the loss of cobalt, it is necessary to set the solubility of cobalt to not less than 100 g-Co/L, and since the solubility product of cobalt is 2.2×10−16, the pH must be pH 6 or less. On the other hand, the solubility product of copper is 2.2×10−20, and since the solubility of copper is 14 mg-Cu/L at pH 6, the separation property of copper is deteriorated.
In addition, it is conceivable to remove copper by a cementation method (substitution method). In the cementation method, metal ions to be removed are reduced by an electrically base metal and removed. Accordingly, copper can be removed from a solution by using a metal less noble than copper. For example, cobalt is a metal less noble than copper, so that when cobalt metal is used, copper in a cobalt chloride solution can be precipitated and removed.
In the cementation method, since the base metal used is ionized and dissolved in the solution, it is necessary to use a metal which does not cause problems even when dissolved. However, since the above-described cobalt metal is a material of a positive electrode material, even if the cobalt metal remains in a cobalt salt solution, it does not affect electrode performance.